How many US nickels make an ounce of silver?

Except for "war nickels", none. All US nickels except those minted from 1942 to 1945 are made of nickel and copper.

"War nickels" contained 1.75 gm of silver instead of nickel because of wartime needs. A troy ounce is about 31.1 gm so you'd need about 18 war nickels to yield an ounce of silver.